The present invention relates to a method and apparatus for pyrolysis of a combustible material, and more particularly relates to a method and apparatus for pyrolysis of a bituminous paving mix.
Bituminous paving mixes, containing aggregate and combustible asphalt binder, are widely used in the road construction industry. Federal and state guidelines require that paving mixes exhibit certain compositional properties to ensure the long-term durability of roadways constructed of such materials. As a result, it is necessary to test the paving mix composition to determine whether the requisite properties are present. For example, it is often necessary to determine the relative amounts of asphalt binder and aggregate, as well as the gradation of aggregate size.
Pyrolysis techniques, wherein the asphalt binder is combusted in order to separate the binder from the aggregate, are known in the art. Such techniques allow measurement of asphalt content, as well as aggregate gradation analysis. Conventional pyrolysis techniques utilize a furnace having electric heater plate elements. Heat energy is produced in the heater plates and transferred to a combustion chamber through conduction and convection. In this manner, the air in the combustion chamber is preheated to an elevated temperature, typically in excess of 500xc2x0 C. Thereafter, a paving mix sample is inserted into the combustion chamber. Once inside the furnace, the paving mix sample is heated by conductive and convective heat transfer to a temperature of about 460-500xc2x0 C. to achieve ignition of the asphalt binder. As the asphalt binder combusts, weight loss is measured using an internal weighing device incorporated into the furnace structure and final asphalt content is determined.
The conventional pyrolysis process has numerous drawbacks. First, the energy transfer from the heater plates to the paving mix sample is highly inefficient and requires the slow process of preheating the air in the furnace to an elevated temperature in order to ignite the paving mix sample. Depending on the type and manufacturer of the furnace, it can take up to several hours to preheat the furnace. Additionally, preheating the oven to ignition temperature and subsequent combustion of the sample can result in extended periods of elevated temperatures within the combustion chamber. These high temperatures can lead to thermal decomposition of the aggregate, resulting in loss of aggregate from the sample and reduction of the aggregate particle size, adversely effecting the accuracy of the analysis. To avoid loss of aggregate, a slow, smoldering type of combustion may be used to maintain a lower combustion chamber temperature. While this is somewhat effective in preventing thermal decomposition of aggregate, the lower combustion temperatures result in increased emission of excess carbon and other potential pollutants. Consequently, conventional furnaces generally require an afterburner to reduce pollutant discharge.
There is a need in the art for a method and apparatus for pyrolysis of a paving mix sample that efficiently transfers heat to the sample and produces relatively low levels of pollutants without the need for an afterburner.
The present invention provides a method and apparatus for pyrolysis of a paving mix sample that combusts the asphalt binder portion of the paving mix in a more efficient manner and with lower pollutant emissions than provided by conventional furnaces. In one embodiment, the present invention utilizes infrared radiation to ignite the asphalt binder and infrared radiation in combination with forced convection to efficiently combust the asphalt binder, while limiting aggregate loss and environmental pollution. Further, by adjusting the temperature of the infrared heater, as well as the air flow through the combustion chamber, the present invention provides a controlled combustion process that is faster and more energy efficient than conventional processes.
The apparatus of the present invention includes an oven having a bottom wall, a top wall, and side walls defining a combustion chamber. A sample support is provided within the combustion chamber for receiving and supporting a sample of paving mix. The apparatus may include a sample container positioned on the sample support and adapted for receiving a sample of paving mix. An infrared heater is mounted within the oven and arranged for emitting infrared radiation towards the sample holder so as to heat the sample of paving mix by means of radiation heat transfer. The apparatus includes both an air inlet and an outlet. The air inlet is positioned for admitting air into the combustion chamber and, preferably, at a location above the sample support. The outlet is positioned for discharging combustion gases from the combustion chamber, preferably at a location below the air inlet to thereby provide for the circulation of air and combustion gases generally downward through the combustion chamber.
In a preferred embodiment, the infrared heater is located in an upper portion of the combustion chamber and is oriented to direct infrared radiation generally downwardly at the sample holder. Further, the air inlet is preferably located in an upper portion of one of the side walls and the outlet is preferably located in the opposite side wall such that airflow in the chamber circulates in a generally downward and side-to-side manner.
A blower is located on the downstream side of the outlet and cooperates therewith for inducing flow into the combustion chamber via the inlet and out of the combustion chamber via the outlet. Preferably, the apparatus further comprises a variable speed blower controller operable for varying the blower speed during pyrolysis of the paving mix. Additionally, a plenum chamber is mounted on the outside of the combustion chamber and communicates therewith via the outlet. An exhaust flue connected to the plenum chamber allows discharge of exhaust gases from the plenum chamber. Preferably, the blower is mounted between the plenum chamber and the exhaust flue. The plenum chamber may include an adjustable air inlet vent for admitting ambient cooling air into the plenum chamber to mix with the exhaust gases entering the plenum chamber via the outlet.
The apparatus may further comprise a housing surrounding the oven and spaced from the top and side walls thereof, and also surrounding the plenum chamber. Preferably, the housing includes am air inlet vent for emitting ambient cooling air into the housing induced by the above-described blower.
A weighing device is operatively connected to the sample support for obtaining weight measurements of the sample during pyrolysis thereof In a preferred embodiment, a processor is operatively connected to the weighing device, and includes an instruction set for calculating weight loss during pyrolysis. In one embodiment, the apparatus comprises an external weighing device adapted for receiving a sample container and obtaining the empty weight and filled weight thereof Preferably, the external weighing device is operatively connected to the processor for communication of the empty weight and filled weight to the processor. The processor preferably includes a first data store for storing the empty weight and a second data store for storing the filled weight. The processor may further include a weight loss correction factor generator operable for calculating a weight loss correction factor.
The present invention also provides a method for assaying the asphalt content of a bituminous paving mix. The method includes placing a sample of a paving mix containing aggregate in a combustible asphalt binder in a sample container. The sample container is placed in a combustion chamber of an oven. The sample is then exposed to radiation from an infrared heater. The paving mix sample is heated by radiation heat transfer from the infrared heater to ignite and combust the asphalt binder. Airflow is introduced into the combustion chamber through an inlet to fuel the combustion of the asphalt binder and combustion gases resulting from the combustion are discharged through an outlet. The process of the present invention further includes adjusting at least one of the temperature of the infrared heater and the rate of airflow through the combustion chamber during the heating step.
In a preferred embodiment, the airflow through the combustion chamber is initially maintained at a relatively low rate to discharge combustion gases through the outlet until the approximate time of ignition of the asphalt binder. Thereafter, the rate of airflow is increased after ignition of the asphalt binder to draw heat into lower portions of the paving mix sample. Additionally, the temperature of the infrared heater can be rapidly decreased while maintaining the increased rate of airflow to cool the aggregate. Preferably, the airflow rate is adjusted by increasing or decreasing the rotational speed (rpm) of a blower operatively positioned to induce a flow of air through the combustion chamber.
In one embodiment, the method according to the present invention further comprises measuring the initial sample weight using a weighing device located external to the oven prior to placing the sample container in the combustion chamber. The initial sample weight is then measured using an internal weighing device operatively positioned within the oven. The sample weight determined by the external weighing device is communicated to a processor operatively connected to the internal weighing device. Thereafter, the two measured sample weights are compared to determine a weight loss correction factor. After combustion of the asphalt binder, the final weight of the paving mix may be measured using the internal weighing device. A corrected final weight may then be calculated by applying the weight loss correction factor to the measured final weight.